[XEND] Massive XendDomain XendDomainInfo reorganisation to use XendConfig.

[XEND] Massive XendDomain XendDomainInfo reorganisation to use XendConfig.

* Added some constants to do with lifecycle management in XendDomain
* Added docstrings
* Made all private functions prefixed with an underscore (_)
* Added lifecycle management in XendDomain which stores configs in SXP
  format.
* Added bits of XenAPI support.
* Moved all XendDomainInfo constants to a separate file
* Moved most of the domain creation login into XendDomainInfo rather
  than in module level functions.
* Moved to use Xen API compatible reporting of state.
* Reorganised methods into logical groups in XendDomain and XendDomainInfo
* Moved all domain configuration validation into XendConfig
* Removed DevController cruft from the bottom of XendDomainInfo to
  XendDevices.

Signed-off-by: Alastair Tse <atse@xensource.com>